An actuator that carries out rotation of a flexible joint by using artificial muscles is known. In this actuator, artificial muscles respectively functioning as an agonist muscle and an antagonist muscle are placed in parallel to each other as a pair, and a wire connected to the agonist muscle and a wire connected to the antagonist muscle are wound around a rotation shaft in a mutually-opposing manner and fixed to the shaft. If the contraction force of the agonist muscle is greater than the contraction force of the antagonist muscle, the agonist muscle contracts greater than the antagonist muscle, and the rotation shaft is rotated toward the side where the agonist muscle is attached. If, on the other hand, the contraction force of the antagonist muscle is greater than the contraction force of the agonist muscle, the agonist muscle contracts greater than the agonist muscle, and the rotation shaft is rotated toward the side where the antagonist muscle is attached.